Chapter 6 – Repetition Statements : Objectives After you have read and studied this chapter, you should be able to Implement repetition control in a program using while statements. Implement repetition control in a program using do-while statements. Implement repetition control in a program using for statements. Prompt the user for a yes-no reply using the showConfirmDialog method of JOptionPane.

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Definition Count-controlled repetitions terminate the execution of the block after it is executed for a fixed number of times. Sentinel-controlled repetitions terminate the execution of the block after one of the designated values called a sentinel is encountered. Repetition statements are called loop statements also.

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter while ( number <= 100 ) { sum = sum + number; number = number + 1; } Syntax for the while Statement while ( ) Statement (loop body) Boolean Expression

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter String inputStr; int age; inputStr = JOptionPane.showInputDialog(null, "Your Age (between 0 and 130):"); age = Integer.parseInt(inputStr); while (age 130) { JOptionPane.showMessageDialog(null, "An invalid age was entered. Please try again."); inputStr = JOptionPane.showInputDialog(null, "Your Age (between 0 and 130):"); age = Integer.parseInt(inputStr); } Example: Testing Input Data Priming Read

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Useful Shorthand Operators sum = sum + number; sum += number; is equivalent to OperatorUsageMeaning +=a += b;a = a + b; -=a -= b;a = a – b; *=a *= b;a = a * b; /=a /= b;a = a / b; %=a %= b;a = a % b;

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Watch Out for Pitfalls 1.Watch out for the off-by-one error (OBOE). 2.Make sure the loop body contains a statement that will eventually cause the loop to terminate. 3.Make sure the loop repeats exactly the correct number of times. 4.If you want to execute the loop body N times, then initialize the counter to 0 and use the test condition counter < N or initialize the counter to 1 and use the test condition counter <= N.

Loop Pitfall - 1 What is the problem with this loop? int count = 1; while ( count != 10 ) { count = count + 2; } 2 2 int product = 0; while ( product < ) { product = product * 5; } 1 1

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Loop Pitfall - 2 Using Real Numbers Loop 2 terminates, but Loop 1 does not because only an approximation of a real number can be stored in a computer memory. Using Real Numbers Loop 2 terminates, but Loop 1 does not because only an approximation of a real number can be stored in a computer memory. float count = 0.0f; while ( count != 1.0f ) { count = count f; } //eight 3s 2 2 float count = 0.0f; while ( count != 1.0f ) { count = count f; }//seven 3s 1 1

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Loop Pitfall – 2a int result = 0; double cnt = 1.0; while (cnt <= 10.0){ cnt += 1.0; result++; } System.out.println(result); 1 1 int result = 0; double cnt = 0.0; while (cnt <= 1.0){ cnt += 0.1; result++; } System.out.println(result); 2 2 Using Real Numbers Loop 1 prints out 10, as expected, but Loop 2 prints out 11. The value 0.1 cannot be stored precisely in computer memory. Using Real Numbers Loop 1 prints out 10, as expected, but Loop 2 prints out 11. The value 0.1 cannot be stored precisely in computer memory

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Loop Pitfall - 3 Goal: Execute the loop body 10 times. count = 1; while ( count < 10 ){... count++; } 1 1 count = 0; while ( count <= 10 ){... count++; } 3 3 count = 1; while ( count <= 10 ){... count++; } 2 2 count = 0; while ( count < 10 ){... count++; } 4 4 Which of these blocks exhibit off-by-one error?

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter do { sum += number; number++; } while ( sum <= ); Syntax for the do-while Statement do while ( ) ; Statement (loop body) Boolean Expression

Control Flow of do-while int sum = 0, number = 1 sum += number; number++; sum += number; number++; sum <= ? true false

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Example: Loop-and-a-Half Control String name; while (true){ name = JOptionPane.showInputDialog(null, "Your name"); if (name.length() > 0) break; JOptionPane.showMessageDialog(null, "Invalid Entry." + "You must enter at least one character."); } Using a break inside loops is generally considered a bad idea.

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Confirmation Dialog JOptionPane.showConfirmDialog(null, /*prompt*/ "Play Another Game?", /*dialog title*/ "Confirmation", /*button options*/ JOptionPane.YES_NO_OPTION);

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Example: Confirmation Dialog boolean keepPlaying = true; int selection; while (keepPlaying){ //code to play one game comes here //... selection = JOptionPane.showConfirmDialog(null, "Play Another Game?", "Confirmation", JOptionPane.YES_NO_OPTION); keepPlaying = (selection == JOptionPane.YES_OPTION); }

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter for ( i = 0 ; i < 20 ; i++ ) { number = scanner.nextInt(); sum += number; } Syntax for the for Statement for ( ; ; ) 1) Initialization 2) Boolean Expression 4) Increment and back to 2) 3) (loop body)

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Control Flow of for i = 0; false number =... ; sum += number; true i ++; i < 20 ?

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter More for Loop Examples for (int i = 0; i < 100; i += 5) 1 1 i = 0, 5, 10, …, 95 for (int j = 2; j < 40; j *= 2) 2 2 j = 2, 4, 8, 16, 32 for (int k = 100; k > 0; k--) ) 3 3 k = 100, 99, 98, 97,..., 1

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter The Nested-for Statement Nesting a for statement inside another for statement is commonly used technique in programming. Let ’ s generate the following table using nested-for statement.

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Generating the Table int price; for (int width = 11; width <=20, width++){ for (int length = 5, length <=25, length+=5){ price = width * length * 19; //$19 per sq. ft. System.out.print (“ “ + price); } //finished one row; move on to next row System.out.println(“”); } INNER OUTER

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter The Formatter Class We use the Formatter class to format the output. Formatter formatter = new Formatter(System.out); format(,,,... ) Example: int num1 = 34, num2 = 9; int num3 = num1 + num2; formatter.format("%3d + %3d = %5d", num1, num2, num3);

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Control Strings Alternatively, we can use : System.out.format(,,, … ) Integers % d Real Numbers %. f Strings % s For other data types and more formatting options, please consult the Java API for the Formatter class.

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Using the Date Class Here's one way to measure the execution time Date startTime = new Date(); //code you want to measure the execution time Date endTime = new Date(); long elapsedTimeInMilliSec = endTime.getTime() – startTime.getTime();

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Problem Statement Problem statement: Write an application that will play Hi-Lo games with the user. The objective of the game is for the user to guess the computer- generated secret number in the least number of tries. The secret number is an integer between 1 and 100, inclusive. When the user makes a guess, the program replies with HI or LO depending on whether the guess is higher or lower than the secret number. The maximum number of tries allowed for each game is six. The user can play as many games as she wants.

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Overall Plan Tasks: do { Task 1: generate a secret number; Task 2: play one game; } while ( the user wants to play );

©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 th Ed Chapter Example Program Write a program to read a list of exam scores (integer percentages in the range 0 to 100) and to output the total number of grades and the number of grades in each letter-grade category (90 to 100 = A, 80 to 89 = B, 70 to 79 = C, 60 to 69 = D, and 0 to 59 = F). The end of the input is indicated by a negative score as a sentinel value. (The negative value isused only to end the loop, so do not use it in the calculations).